The present disclosure relates to level shifting circuits for shifting a voltage from one voltage level to another level and, more particularly, to level shifting circuits for shifting a time-varying input voltage.
In analog circuits, input and output voltages can be any value in a range. In contrast, the input and output voltages in digital logic circuits are required to be one of two values corresponding with a logic one or a logic zero. For example, five volts may correspond with a logic one and zero volts may correspond with a logic zero. This example is a simplification because precise voltages are not readily attainable in practical circuits and some tolerance is allowed. As an example, any voltage between 3.5 and 5.0 volts might correspond with a logic one and any voltage between 0.0 and 0.8 volts might correspond with a logic zero. The voltage ranges for logic levels can also be expressed in terms of a supply voltage. For example, a voltage that is 70 to 100 percent of the supply voltage might correspond with a logic one, while a voltage that is zero to 16 percent of the supply voltage might correspond with a logic zero. Different logic families and different technologies use different supply voltages. It is common in the art to refer to the voltage levels for a particular logic family and technology using a single value, i.e., the voltage level corresponding with the logic one. For example, a digital logic circuit may be described as 5V, 3.3V, or 1.7V logic.
Known circuits use a variety of voltage levels. Circuits using new voltages levels are introduced from time to time. A first digital logic circuit that uses a first voltage level can have a need to communicate with a second digital logic circuit that uses a second, different voltage level. Accordingly, a circuit at the interface between the first and second digital logic circuits that shifts the voltage levels used by one circuit to the voltages levels used by the other circuit enables the two circuits to communicate.
Digital logic circuits may use metal oxide semiconductor field-effect (MOSFET or MOS) transistors. MOSFET transistors may be n-channel MOSFETS (NMOS) or p-channel MOSFETS (PMOS). In addition, digital logic circuits may use complementary metal oxide semiconductor (CMOS) technology. In CMOS technology, both NMOS and PMOS transistors are present.